NiMH Battery Chargers

NiMH batteries are very sensitive to overcharging. Thus if you want
to keep your batteries working as long as possible, you should avoid
overcharging. There are two basic ways of doing this. One is to
discharge your battery to an almost empty state (which is determined
by the voltage of the battery dropping to a certain predefined limit,
usually about 1 volt per cell), and then charging it up using a
timer. This avoids both overdischarge, which leads to
cell reversal, as well as
overcharging.

If you don't have the equipment to precisely discharge your
battery to that almost empty state, then what you should do it just
recharge it after use, using a smart charger to prevent overcharging.
A smart charger charges the battery at a reasonably high rate (one amp
or more). When it detects that the battery is full, it turns
off. Well, it doesn't turn off, it just switches to a low trickle
current to counter self-discharge. Some people argue that since NiMh
are so easily damaged by overcharging, the charger should really turn
off completely, but occasionally let in a burst of current to counter
self-discharge. However, I only know of one smart charger that does
this (the Specialized one below).

There are two features of NiMH batteries that are used to
terminate smart charging. One is that as the battery approaches being
full, the temperature starts going up, and the temperature continues
to increase as time goes on. If the current isn't turned off, the
battery will overheat and be destroyed. Another feature is that the
battery voltage slowly increases, and just after the battery reaches
100% capacity, the voltage drops by a small amount.

From reading NiMH battery data books, I found out that by far the
best way to determine when a battery is full is dT/dt. This requires
you to keep track of the temperature of the battery. When the
temperature has gone up by a certain amount over a given interval, the
battery is full. I seem to recall 1.5 degrees C over 2 mins, but don't
hold me to that. I don't know of a chip or off the shelf charger that
does this, and it requires a thermistor built into the battery
pack.

I have a Lumicycle
lighting system with NiMH batteries. Lumicycle offer a smart charger
with their system as an upgrade, and I naturally got this one. It
works by detecting the slight voltage voltage drop in a full
battery. The company that makes this charger makes a very similar one
for NiCad batteries, but their NiMH charger terminates fast charging
after detecting a much lower voltage drop than the NiCad ones. It
seems to work. That is, it charges for less time when I've used only a
small part of the battery capacity, and for longer when I've used
more.

However, according to the data books, this algorithm
turns off the fast charge after a small amount of overcharging has
already occurred, and hence a very small amount of damage is being
done to the battery with every charge. The data books that I read
suggest that you'll get a 20% fewer charge cycles from your NiMH
batteries doing this. However, it is a far better approach than using
a slow charger, like the 16 hour charger as supplied with the standard
Lumicycle system. With that kind of stupid charger, it's entirely luck
as to whether you undercharge it, get it just right, or overcharge
it.

If you are really into DIY, one route to getting a NiMH smart
charger is to make it from a Maxim 712 chip. Maxim makes two very
similar charger chips. The MAX 713 is for NiCad batteries, and the MAX
712 is for NiMH. The 713 turns off when the voltage drops after NiCad
battery is fully charged. The 712 turns off when the voltage levels
out rather than dropping. They felt that this was a safer approach
than trying to detect the small drop that occurs. I think it makes
sense, especially as it terminates charging a bit quicker, resulting
in less overcharging. The MAX 712 also monitors temperature, and
charging can be terminated by maximum temperature. But this requires
you to make up battery packs with thermistors in them. For more info,
check out Maxim's Web pages.
You can get ahold of a data sheet by looking up the 712 in this
chart.

By far the best charging system seems to be the one included with
the soon-to-be-released Specialized Fireball Team and Comp lights.
According to the developer of these lights, their battery technology
is essentially the same as that used by the laptop computer
industry. Basically, the battery pack includes a "gas gauge" and some
very smart electronics which monitor the state of the battery based on
battery voltage, elapsed time, and ambient temperature. When plugged
into the charger, fast charging starts. The cutoff for fast charging
is determined as a combination of time, voltage, and temperature.
When the pack is fully charged it does not trickle charge, it turns
off. The gas gauge then begins to monitor self-discharge. The gas
gauge will then occasionally turn the charger on to top-off the pack.
This eliminates the chance of over-charging which is the leading cause
of damage to NiMH dells.

An alternative to a smart charger is to make a trickle
charger. However, since NiMH batteries are so very sensitive to
overcharging, the best approach is to use a very low current in order
to minimize overcharge damage. One frequent poster to the
bikecurrent mailing
list uses a charger that charges at a rate something like C/60 and
has had good results with it. C/60 is the capacity of the battery in
amp-hours divided by 60. So if you had a battery with capacity 4
amp-hours (4000 mAh), C/60 is 67 mA. At this current rate, the battery
would be charged from empty in about 84 hours, i.e. about 3 and a half
days. If you left it plugged in longer than that, it wouldn't matter
much because of the low charge rate. Obviously this system cannot be
used when you require your battery to be charged up quickly!